‘Leader image’, according to Worcester "accounts for between a quarter and a third of the forces that might cause the swing voter to consider moving from one party to another."

Economic competence is another point of the triangle and was popularised in 1992 when James Carville, Clinton election stategist, made 'It's the economy, stupid' the defining motto of his election war-room.

Party unity is the third point of the triangle. Unity was once the Tory party's secret weapon. Margaret Thatcher appointed the centrist William Whitelaw as her deputy in the early years of her premiership as part of her determination to keep the party balanced. But the party became increasingly difficult to manage and unity collapsed after the regicide of 1990. Under David Cameron the party has begun to look united again. All three of his immediate predecessors are loyally supportive of the Tories' fourth Leader of the Opposition since 1997 and as figures as diverse as John Redwood, Peter Lilley, Michael Heseltine and Ken Clarke are helping formulate future policy.